1. Field of the Invention
This invention pertains to a system and method for maximizing the combustion efficiency of a burner supplied with fuel and air while minimizing the production of noxious by-products. More specifically, the invention involves the use of sensors adapted to detect infrared emissions from molecules such as CO.sub.2, H.sub.2 O, CO, and HCl which may be present in the elevated temperature of the exhaust gas stream downstream but in close proximity to the burner. The information obtained from the sensors is relayed to a computer (microprocessor). The computer calculates various ratios of the detected constituents which it compares with information stored therein concerning how those ratios vary with respect to the air-fuel ratio fed to the burner. Depending upon the relationship between the sensed information and the stored information, the microprocessor provides an output directing a process controller to increase, decrease, or make no change in the ratio of fuel to air supplied to the burner.
The invention is useful for electric power plants (natural gas or coal fired) hazardous waste incinerators, various kilns and municipal waste incinerators. In many cases problems exist with changes in the fuel composition and with accurate fuel delivery (e.g., coal, municipal waste). Use of the invention with internal combustion engines is also contemplated.
2. Background of the Prior Art
Several issued patents discuss infrared absorption measurements on combustion (or other) products. Those measurements involve the use of an infrared source module and the gases to be analyzed pass between the infrared source (ceramic glow-bar) and a suitable detector. The present invention measure the radiation emitted by a molecule as it undergoes a transition from a higher to a lower quantum vibrational-rotational level. Thus, the present invention involves a passive measurement which requires that the molecules of the products of combustion to be detected (such as H.sub.2 O, CO.sub.2, and CO) be present at an elevated temperature in the exhaust gas eminating from the burner and act as their own source of emitted radiation.
The following documents, contrary to the present invention, all employ an absorption configuration to detect and measure various components of a gas stream: Astheimer (U.S. Pat. No. 3,735,127); Baskins et al. (U.S. Pat. No. 4,549,080); Day (U.S. Pat. No. 4,891,518); Conlon et al. (U.S. Pat. No. 4,914,719); Lee et al. (U.S. Pat. No. 4,990,780); Johnson et al. (U.S. Pat. No. 5,021,662); Butler et al. (U.S. Pat. No. 5,138,163); and Coe et al. (U.K. patent document 2,035,552). When using an absorption configuration, an infrared source is required and in some cases it is also necessary to extract a gas sample from the combustion gas stream. Moreover, an absorption measurement is only useful if the infrared source temperature is considerably higher than the gas stream.
The patent to Gulitz et al. (U.S. Pat. No. 4,059,385) discloses a device which measures the flame temperature and uses that temperature as a basis for adjusting the air fuel ratio for the combustion of oil or natural gas. In Gulitz et al., unlike the present invention which provides sensors for measuring molecular emissions, the sensors are used to measure background radiation.
Spahr (U.S. Pat. No. 4,419,918) is directed to control of the air flow to a burner on the basis of CO.sub.2 measurements using some type of probe in the exhaust stack. The details of the probe are not set forth in the patent. Test results conducted by the inventors of the present invention do not indicate that CO.sub.2 monotonically changes over the full range of air to fuel or stoichiometry values. Accordingly, CO.sub.2 measurements would be of limited value as a parameter for adjusting the ratio of air to fuel supplied to a burner. In addition, placement of the probe in the exhaust stack results in an undesirable time lag between the combustion event which produced the CO.sub.2 combustion product detected by the probe and the subsequent adjustment of the air to fuel ratio.
Frish et al. (U.S. Pat. No. 5,112,215) describe a process which attempts to quantify the emitted radiation arising from entrained high temperature particles. Particle temperatures are determined based on the Planck function and numerous simplifying assumptions. Particles must be present for these measurements, the method apparently is not applicable to natural gas, oil fired, and hazardous waste fired combustion.
Beer et al. (U.S. Pat. No. 5,206,176) describes a process which involves laser induced fluorescence measurements on polycyclic aromatic compounds. The emissions produced by fluorescence are weak and difficult to detect. In addition, the measurements taken in the Beer et al. process are done in the visible portion of the electromagnetic spectrum. The patent does not discuss the adverse effect particle scattering will have on the process disclosed therein when using a fuel such as coal.
Bonne et al. (U.S. Pat. No. 4,913,647) describe a process involving the use of emissions from, in most cases, reactive radical intermediates in the transformation of fuel to CO.sub.2, H.sub.2 O, CO, etc. Unlike the present invention, those measurements are taken in the ultraviolet and visible portion of the electromagnetic spectrum where particle interference must be large--especially for a pulverized coal combustion. The emitted radiation is not modulated or chopped by a mechanical device in the process disclosed by Bonne et al. and thus flame flicker will be present. That flicker will cause a low frequency modulation to be observed in the signal produced by the sensing device. The accuracy of the four element silicon detector located adjacent the base of the flame would likely be adversely affected by soot collecting on the window of the detector. A decrease in intensity of one of the measured constituents of the combustion process due to soot particles on the window of the detector may not be the same as for another measured constituents. In this respect, we note that the scattering of light scales as X/2, thus a 1 .mu.m particle will attenuate or scatter more light at 195 nm (CO) than at 432 nm (CH).
Clark et al. patent (U.S. Pat. No. 5,112,217) and the publication (Smith, A.; Sutherbank, J.; Taylor, D. S.; "Fuel: Air Ratio Control Using Infrared Spectroscopy," Inst. Fuel p. 59 June (1975) both involve a curve of infrared emission intensity against stoichiometry values (.phi.). The curve resembles a parabola with two, non-unique, values of .phi. possible for 1 value of infrared emission intensity. In the present invention there is a unique value of infrared emission intensity for each stoichiometry value.